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Rock Mechanics.pdf - Mining and Blasting

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Figure 5.6 (a) Core drilling a slot for<br />

a flatjack test <strong>and</strong> (b) slot pressurisation<br />

procedure.<br />

PRE-MINING STATE OF STRESS<br />

measurement system seeks to restore the original, post-excavation local state of stress<br />

at the experiment site. Such methods are intrinsically more accurate than those relying<br />

on positive disturbance of the initial condition whose state is to be determined.<br />

Three prerequisites must be satisfied for a successful in situ stress determination<br />

using flatjacks. These are:<br />

(a) a relatively undisturbed surface of the opening constituting the test site;<br />

(b) an opening geometry for which closed-form solutions exist, relating the far-field<br />

stresses <strong>and</strong> the boundary stresses; <strong>and</strong><br />

(c) a rock mass which behaves elastically, in that displacements are recoverable<br />

when the stress increments inducing them are reversed.<br />

The first <strong>and</strong> third requirements virtually eliminate the use as a test site of an excavation<br />

developed by conventional drilling <strong>and</strong> blasting. Cracking associated with<br />

blasting, <strong>and</strong> other transient effects, may cause extensive disturbance of the elastic<br />

stress distribution in the rock <strong>and</strong> may give rise to non-elastic displacements in the<br />

rock during the measurement process. The second requirement restricts suitable opening<br />

geometry to simple shapes. An opening with circular cross section is by far the<br />

most convenient.<br />

The practical use of a flatjack is illustrated in Figure 5.6. The jack consists of<br />

a pair of parallel plates, about 300 mm square, welded along the edges. A tubular<br />

non-return connection is provided to a hydraulic pump. A measurement site is established<br />

by installing measurement pins, suitable for use with a DEMEC or similar<br />

deformation gauge, in a rock surface <strong>and</strong> perpendicular to the axis of the proposed<br />

measurement slot. The distance d0 between the pins is measured, <strong>and</strong> the slot is cut,<br />

using, for example, a series of overlapping core-drilled holes. Closure occurs between<br />

the displacement measuring stations. The flatjack is grouted in the slot, <strong>and</strong> the jack<br />

pressurised to restore the original distance d0 between the displacement monitoring<br />

pins. The displacement cancellation pressure corresponds closely to the normal stress<br />

component directed perpendicular to the slot axis prior to slot cutting.<br />

Determination of the field stresses from boundary stresses using flatjacks follows<br />

a procedure similar to that using strain observations. Suppose a flatjack is used to<br />

measure the normal stress component in the direction OA in Figure 5.5c, i.e. the<br />

plane of the flatjack slot is perpendicular to the axis OA. If A is the jack cancellation<br />

152

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